A molten metal can be coated on a substrate, e.g., a continuously moving substrate (a metal strip), in a vacuum environment through known evaporation methods.
For example, techniques for changing a solid coating material into vapor by heating and vaporization and coating (evaporating) the vapor on a substrate are classified depending on a heating method. Representative examples of vacuum evaporation techniques include thermal evaporation, electron beam evaporation, electromagnetic levitation evaporation, and so on.
As for the thermal evaporation technique, a coating material is resistively heated and vaporized and is then coated on a substrate. However, the thermal evaporation technique has a limit to the heating of the coating material using resistance heating. Since it is difficult to coat high melting point materials, such as titanium (Ti) or chromium (Cr) with the thermal evaporation technique, this technique has been applied to the coating of low melting point materials as zinc or magnesium (Mg).
In addition, the thermal evaporation technique has a limit to a coating speed thereof. For example, the coating speed of magnesium is a mere 30 μm·m/min.
As for the electron beam evaporation technique, a solid coating material is loaded into a crucible and is locally heated by an electron beam. In this manner, high melting point materials are vaporized and coated. However, heat loss caused by contact between the vaporized material and the crucible reduces the energy efficiency and coating speed thereof. For example, the coating speed of aluminum is merely 20 μm·m/min.
Meanwhile, other evaporation techniques are disclosed in PCT International Publication No. WO 2006/021245 (Korean Patent Application No. 2007-7006446) or U.S. Patent Publication No. 2005/0064110.
Specifically, a coating material is heated to a levitation state through an alternating electromagnetic field which is generated when a high-frequency alternating current is applied to an electromagnetic coil enclosing the coating material. Thus, metal (coating) vapors are generated and coated on a substrate, without heat loss caused by a crucible.
However, the PCT International Publication (the Korean Patent Application) suggests only the basic concept of coating through levitation and heating by the alternating electromagnetic field. In the case of the U.S. Patent Publication, a substrate is coated in such a state that a vaporization space is vacuum-isolated. However, since a coating material is supplied through a solid wire feeder, a supply path (position) of a wire, which is a coating material, is limited to only a lateral side (a horizontal direction).
Furthermore, in the case of the U.S. Patent Publication, a solid wire is supplied to a space where an alternating electromagnetic field is formed, and is then levitated and heated. Thus, as compared to a liquid material, the speed of coating by the solid wire is reduced or the cost thereof is increased due to the increase in heating load.
An aspect of the present invention provides a continuous coating apparatus which can perform a high-speed coating on a substrate (a metal strip) which is a moving coating target, and in particular, can supply a liquid coating material (a molten metal) to a levitation-heating space through various paths.
An aspect of the present invention also provides a continuous coating apparatus which can easily control a supply flow rate of the liquid coating material, can improve coating workability and precision, and has a simplified structure.